We propose to continue using multiparameter sorting and analysis with the Fluorescence Activated Cell Sorter (FACS) both to define functionally and clinically relevant lymphocyte subpopulations in mouse and man and to select variant and DNA-transformed animal cell lines for recombinant DNA analyses of the genes and genetic mechanisms that control the structure and production of immunologically relevant molecules. Thus, we plan to: (1) Extend our current library of immunoglobulin constant-region (IgCH) "switch-variant" families derived from hybridoma cell lines by selecting interchromosomal and interspecies (mouse-human) variants that produce "chimeric" antibodies, variants that produce modified parental IgCH structures, and regulatory variants that show changes in the expression of parental IgCH genes. We will then investigate the genetic mechanisms underlying these "IgCH switches" by testing for mutations, rearrangements, somatic cross-overs or other chromosomal changes; methylation or other DNA modifications affecting transcription; or differential splicing of primary transcripts or other mechanisms affecting translation. (2) Transform with DNA to obtain cell lines that secrete or carry products of genes derived from a different species or haplotype. In particular, we hope to obtain transformants for genes expressed in differentiated lymphocytes, e.g., mouse Lyt-1 and Lyt-2 and their human homologs (Leu-1 and Leu-2). (3) Define B-\and T-cell subpopulations in mouse and man according to correlated surface marker expression and to investigate the genetic mechanisms influencing the expression of surface markers, secreted products and differentiation-specific mRNAs in FACS-isolated T-\and B-cell subpopulations.